This invention relates to rewritable optical recording media in which information can be recorded, reproduced and erased by using light and heat, a method of information recording, reproducing and erasing in said optical recording media and units therefor.
In optical information recording and reproducing units, information is recorded or erased by means of a change of optical characteristic (energy reflectivity: to be referred to as "reflectivity" hereinbelow) according to the phase-change between the amorphous phase and the crystalline phase caused when a laser power is changed for irradiation. Various methods for optical information recording and reproducing have hitherto been proposed. As shown in the following known examples, the reflectivities of conventional optical disks are smaller in the amorphous state corresponding to record than in the crystalline state corresponding to erasion. Known examples: Chemistry and Industry, Vol. 39, No. 3, 1986, P. 174; a disk using a Te.sub.87 Ge.sub.8 Sn.sub.5 recording material, described in Applied Physics Letters, Vol. 46 (8), 1985, P. 735; a disk using a TeOx.Ge or TeOX.Sn recording material, described in National Technical Report, Vol. 29, No. 5, P. 731; a disk using an Sb.sub.2 Se recording material, described in Appl. Phys. Letters Vol. 48 (19), 1986, P. 1256; a disk using a GeSbTe recording material, described in Proc. International Symposium on Optical Memory, 1987, Japanese Journal of Applied Physics, Vol. 26, 1987, Supple 26-4, P. 64; a disk using a Te.sub.44 Ge.sub.16 Se.sub.10 Sb.sub.28 recording material, described in Technical Report of Electronic Information Communication Society, Shingaku Giho, Vol. 87, No. 310CPM 87-88, 1987, P. 26; FIGS. 5 and 7 of Technical Report of Electronic Information Communication Society, Shingaku Giho, Vol. 87, No. 310, CPM87-90, 1987, P. 40; a disk using an SeTe-Se recording material, described in Proc. SPIE529, 1985, P. 46; a disk using an InSeTl recording material, described in Appl. Phys. Letter, vol. 50, 1987, P. 668; a disk using a TeGeSn recording material, described in J. Appl. Phys. 60 (12), 1986, P. 4320.
As an exceptional example, FUJITSU. 38, 2, 1987, page 144 describes a disk using an SeInSb recording material as a disk having a higher reflectivity in the recording state than in the erasing state. This disk utilizes crystal I and crystal II, which have different crystalline structures.
The following are known examples of erasing methods for the disks having a smaller reflectivity in the amorphous state than in the crystalline state as mentioned above. In the case of an Sb-Te-Ge recording material described in Extended Abstracts (The 35th Spring Meeting, 1988); The Japan Society of Applied physics and Related Societies, page 839, the erasing is carried out by irradiating a recorded bit with an erasing beam, in which method a coarse crystal adjacent to an amorphous part grows within the bit to recrystallize the amorphous part. That is, the erasing is effected by a method of recrystallizing the recorded bit by heating it to a temperature at which the recorded bit is not melted (to be referred to as "solid phase transformation" hereinbelow). However, it is pointed out that the number of incompletely erased parts is large. In the case of an SbSeTeGe recording material described in Extended Abstracts (The 35th Spring Meeting 1985); the Japan Society of Applied physics and Related Societies 28P-ZQ-12, 1988, page 842, the single beam overwrite method is used. The overwrite method is classified into two erasing ways. One is that the erasing is carried out by crystallizing an amorphous part in the solid phase state. In this erasing way also, the erasability is -25 dB, and incompletely erased parts still remain (solid phase transformation). In the other way of erasing a recorded bit by melting the recorded bit and crystallizing it in a solidification process (to be referred to as "liquid phase transformation"), the erasability is -15 dB as shown in FIG. 2, and the number of incompletely erased parts is larger in this way than in the solid phase transformation. In the case of Sb.sub.2 (Te-Se).sub.3 -GeTe described in Technical Report of Electronic Information Communication Society, Shingaku Giho, Vol. 87, No. 310, CPM87-90, 1987, page 41, the erasing method uses the solid phase transformation. However, the erasability with a laser power of 10 mW is about 30 dB as shown in FIG. 3. When the laser power is 15 mW or more, the liquid phase transformation takes place, and the erasability becomes high. It is pointed out, however, that the high erasability is because unrecorded parts (crystal parts) are melted by irradiation with a laser power of 15 mW or more melts to become amorphous but it is not because recorded bits are erased.
In the case of TeGeSeSb described in Technical Report of Electronic Information Communication Society, Shingaku Giho, Vol. 87, No. 310, CPM 87-88, 1987, page 27, a high erasability is obtained, which high erasability is due to its double beam method. That is, a recorded film is melted with one circular beam of the two, and the remaining one is an erasing-use beam. As shown in FIG. 4, the errasing method comprises increasing the laser power once [to not less than Tm (melting point)] to melt a recorded spot, and then, in the course of cooling, irradiating an elliptic laser spot beam in a power of not less than Tx (crystallization temperature) and not more than Tm, and this method gives an erasability of -40 dB.
As discussed above, when the erasing is carried out in disks having a smaller reflectivity in the amorphous state than in the crystalline state, the problem is that the non-erased part remains in the cases of both the solid phase transformation and the liquid phase transformation, and any high erasability cannot be achieved. In addition, the double beam method gives a high erasability, but requires a complicated unit therefor, which requirement is a problem. The optical properties (refractive index n, extinction coefficient k) of known recording films are as follows (see TABLE 1).
TABLE 1 ______________________________________ Optical properties Recording film n k ______________________________________ Te.sub.80 Se.sub.10 Sb.sub.10 Amorphous 4.0 1.3 4.6 2.3 Crystalline GeSb.sub.2 Te.sub.4 4.7us 1.3 6.9 2.6 Crystalline 3.8 0.8 Amorphous 5.6 1.2 Crystalline GeTe 4.4 1.1rphous 5.4 1.7 Crystalline Sb.sub.2 Te.sub.3 5.0hous 2.7 5.3 5.8 Crystalline ______________________________________
In Te.sub.80 Se.sub.10 Sb.sub.10 described in J. Appl. Phys. 59. (6), 1986, page 1819 (n and k in the amorphous state are abbreviated as n amo and k amo, respectively, and n and k in the crystalline state are abbreviated as n cry and k cry, representatively), n amo=4.0, k amo=1.3, n cry=4.6 and k cry=2.3. In GeSb.sub.2 Te.sub.4 described in Proc. Int. Symp. on Optical Memory, 1987, page 62, n amo=4.7, k amo=1.3, n cry=6.9 and k cry=2.6. In TeOx described in Proc. Int. Symp. on Optical Memory, 1987, J. JAP. , Vol. 26, 1987, Supple. 26-4, page 57, n amo=3.8, k amo=0.8, n cry=5.6 and k cry=1.2, and in GeTe described therein, n amo=4.4, k amo=1.1, n cry=5.4, k cry=1.7. In Sb.sub.2 Te.sub.3 described in Cellection of Manuscripts for No. 35 Applied Physics Associated Joint Lectures, 1988, 28P-ZQ-5, page 840, n amo=5.0, k amo=2.7, n cry=5.3 and k cry=5.8.
As is clear from the above, the refractive indices and extinction coefficients of the known examples are all in the relationships of n amo &lt;n cry and k amo &lt;k cry, and in particular, the refractive indices in the amorphous state are characteristically smaller than those in the crystalline state.
As stated hereinabove, in the existing rewritable optical recording media in which information is recorded and erased by using the phase-change between amorphous and crystalline phases, incompletely erased signals remain to a large extent, and the reduction of the incomplete erasion to a practical-use level is now the greatest issue in development. Further, if a single laser beam can be used to overwrite new signals while erasing recorded signals, the data transmission speed is 2 to 3 times as fast as the speed of conventional media, and the optical pick-up therefor can be considerably simplified as compared with one using a plurality of laser beams. Then, great effects are thus achieved on the practical use of optical recording media.